Coaxial high frequency conductor and process of its fabrication



July 10, 1956 D COAXIAL HIGH FREQUENCY CONDUCTOR AND PROCESS OF ITS FABRICATION Filed Sept. 20, 1952 Inventor-z Dallas T. Hum-d, b5 4.1

His Attorne g.

COAXIAL HIGH FREQUENCY CONDUCTOR AND PROCESS OF ITS FABRICATION Dallas T. Hurd, Burnt Hills, N. Y., assignor to General Electric Company, a corporation of New York Application September 20, 1952, Serial No. 310,579

3 Claims. (Cl. 174-102) The present invention comprises an improved high frequency conductor of the type commonly known as coaxial cable which comprises concentric spaced conductors and an insulating material filling the space between such conductors. It is the object of my invention which comprises both a process and the resultant product to provide improved coaxial cables which are capable of operating without excessive corona loss at higher impressed voltages and under more severe physical conditions than was possible with similar coaxial cables as formerly constructed.

in accordance with my invention, at least the inner or enclosed conductor, and in some cases both of the concentric conductors, are provided with an adherent film of an oxide of copper of the kind produced either by chemical oxidation in an aqueous alkaline solution or, preferably, by electrolytic oxidation in an aqueous alkaline solution. For convenience, copper oxide produced by either of these two methods will be referred to herein generically as solution-oxidized copper.

The accompanying drawing illustrates in Fig. l a somewhat conventionalized side view of an electrolytic apparatus for producing a film of oxide on wire, and Fig. 2 shows a coaxial cable embodying my invention.

The drawing illustrates in Fig. 2 a section of a length of coaxial cable including an outer enclosing tubular conductor 4 which conveniently consists of braided or woven conductive material and an inner enclosed axial conductor 3 which, as illustrated, may consist of a bundle of relatively small gauge wires, or may comprise a solid conductor. The provision of multiple fine conductors provides desired flexibility, the wires of smaller gauge in close juxtaposition constituting in elfect a single conductor. The central axial conductor 3, either single or multiple, ordinarily consists of copper although it may consist of a non-cupreous metal, as, for example, aluminum or iron, which is provided with a coating of copper. The external coaxial conductor 4 may consist of braided small-gauge wires, as described in Saflford, U. S. Patent 2,379,318. This external conductor 4 also consists of copper, or copper-coated non-cupreous metal. The intermediate insulation 5 may consist of polymerized organic plastic composition as, for example, polymerized ethylene (Polythene), polystyrene, polytetrafiuoroethylene (Teflon), or of various compositions containing sulphurfree rubber, as described in the previously mentioned Satiord patent. An external protective sheath 6 commonly is provided for coaxial cable.

in accordance with the improvement eilected by my invention, the conductors of coaxial cable and in particular the individaul wires of the inner conductor 3 are oxidized by either one of the following solution methods, preference being given to Method II.

Method I.-The individual wires of the stranded axial conductor 3 are oxidized by contact with an alkaline, aqueous solution containing an active oxidizing agent. An oxidizing reagent of this type is disclosed in Myer, U. S. Patent 2,364,993, wherein is described a reagent nited States Patent "ice mixture comprising sodium hydroxide and sodium chlorite. An oxidizing reagent of this type is sold in the market as Ebonol C by Enthone, inc, of New Haven, Connecticut. A suitable concentration of such oxidizer is 25 grams per 100 cc. of water (or 1% lbs. per gallon of water). Copper to be oxidized having chemically clean surface is placed in contact with such solution at l00 C. for about one-half hour whereby a dark olive-drab nearly black oxide is produced on the copper. This oxide comprises both cuprous and cupric oxide and also hydrous oxide of both forms.

Method ll.-Wires, or other articles of copper, may be oxidized anodically in an aqueous alkaline solution. Preferred methods of anodic oxidation are described in my copending applications, Serial No. 310,576, filed concurrently herewith, and Serial No. 310,578, filed concurrently herewith.

As described more fully in my application, Serial No. 310,576, and as indicated in Fig. l, a copper wire or cable may be oxidized by progressively and continuously causing such wire to travel through a series of electrolytic cells 7 each containing a concentrated aqueous solution of sodium hydroxide, or equivalent electrolyte, and during such progression of the wire subjecting said wire to electrolysis, the wire functioning as anode, and the metal cell container as cathode. The electrolyte may have a concentration of about 20% and may be maintained at a temperature of about 98 to 99 C. The potentials suitable for oxidation ordinarily should rise from approximately 0.4 volt in the first cell to a value of about 1.3-2.0 volts in the last cell, the intermediate voltages rising progressively from 0.4 volt to about 1.3 volts. Regulation of voltage is carried out by suitable resistances 88, leading to the respective cells from the supply conductor 9, the circuit being completed by a conductor 10. Heating of the electrolyte is accomplished in a circulatory conduit ii which includes a tank 12 containing a heater 1?. A pump 14 circulates the electrolyte in the conduit 11 which also includes a flow meter 15. The Vent pipes 16 provide for discharge of any gases that may be evolved during the anodic oxidation of the wire.

The method of applying the particular thermoplastic insulating layer to the oxidized wire to get the intimate adherence desirable for reduction of corona loss and for the other desirable advantages of my invention comprises contacting the wire, after it has been properly oxidized, Washed and dried, with heated thermoplastic material. in a molten or semi-fluid condition, the thermoplastic fiows into intimate contact with and wets the copper oxide surface of the wire. Such process may be carried out on an extrusion machine, such as is normally used for extruding thermoplastic insulating sheaths on electrical conductors, when anodized copper wire is passed through the machine to be coated with polyethylene plastic. If the insulating sheath in the coaxial cable is to comprise a rubber base composition such as described in Safi'ord, U. S. Patent 2,379,318 of June 26, 1945, the unvulcanizcd elastomeric composition may be so extruded onto the wire that the composition surrounds and wets the oxidized surface of the conductor, then is vulcanized into an elastomeric coating by the application of heat. it is to be emphasized that an important part of the process whereby a firm and intimate adhesion of the insulating sheath to the conductor is achieved is the wetting of the surface of the oxidized wire by the insulating material while molten, or before vulcanization as the case may be. Finally the coaxial braided electrode 4 and the external protective sheath 6 are applied.

Ordinarily, a thickness of oxide film equivalent to from 0.3 to 2.0 milligrams of copper oxide per square centimeter of wire surface is adequate to give firm bonding although satisfactory bonding may be obtained with some materials on thicknesses of oxide below or above these limits. Such coatings can be produced by the electrolytic anodic oxidative process in from 30 seconds to minutes per segment of wire so anodized; depending upon process conditions.

Coaxial cable as, for example, a cable identhied. as Amphenol Co. RGStS/U comprising unoxidized conductors and; polyethylene dielectric, and having a construction similar to the coaxial cable described in Salford, U. S. Patent 2,379,318, begins to have appreciable corona loss, at a potential of about 1.2 kilovolts and at about 1.4 kilovolts a steady somewhat increased corona is observed. Similar cable having the inner conductor oxidized by either of the above methods, conveniently by Method: 11,, can be operated. at about 100% higher voltages before appreciable corona loss appears, specifically at about 2.5 kilovolts and, steady corona is not observed at voltages below about 3.0 kilovolts. The advantages of the copper oxide coating on the. central conductor are even more aparent when the cables are subjected to bending stresses or other deformations, such as coaxial cables frequently are subjected to in normal usage. With the usual bare copper inner conductor and non-adherent polyethylene insulating sheath, separation of thesheath from the conductor occurs on sharp. bending and, as a consquence, increased corona occurs in the void thus formed, resulting in more rapid deterioration of the cable under electrical stresses. For example, Amphenol RG8/U coaxial cable (or cable such as shown in U. S. Patent 2,379,318) when sharply deformed exhibited a corona intensity of 16 micromicrocoulombs at 4.0 kilovolts. Thisv is a high corona intensity which leads rapidly to dielectric breakdown and failure of the cable. In contrast to this, a sample of cable constructed to. the same dimensions as RGS/U and subjected to the same deformation treatment exhibited only 0.3 micromicrocoulomb at 5.0 kilovolts and, in fact, could be operated at 15.0 kilovolts for extended periods without breakdown or excessive corona losses that normally would lead to rapid failure.

Cables embodying my invention also are more moistore-resistant than cables containing non-oxidized copper conductors.

These differences in function I believe to be due to the fact that the intermediate insulation 3, for example, polyethylene is bonded by fusion to the oxidized copper surface and grips the oxidized surface so closely that the existence of minute air films adjacent the copper conductors is precluded. It is observed, for example, that polyethylene sealed by fusion to a copper surface, oxidized in the above-described manner and having a mini mal thickness of oxide layer corresponding to ca. 0.3 mg./cm. is so firmly adherent that the polyethylene cannot be removed from the surface. Attempts to remove the plastic film result in a rupture of the plastic and the retention of a surface layer of plastic remaining firmly adherent to and embedded in the oxide layer.

What I claim as new and desire to secure. by Letters Patent of the United States is:

1. An electric cable comprising the combination of an electric conductor at least a surfacev layer of which consists of copper, said copper layer having provided thereon an adherent coating of oxide of copper produced by the anodic oxidation of copper in a alkaline solution, a spaced enclosing conductor surrounding said oxidecoated conductor and an electric insulator comprising polymerized ethylene filling the space between said conductors.

2. A coaxial conductor comprising spaced electric conductors, one of which is an axial conductor enclosed by a cooperating electrode, said axial conductor having provided thereon an adherent coating of oxide of copper produced by oxidation of copper in an alkaline solution and a polymerized hydrocarbon plastic composition filling the space between said conductors, and being adherently united with said oxide-coated axial conductor.

3.. A coaxial conductor comprising spaced electric conductors, one of which is an axial conductor enclosed by a cooperating electrode, said axial conductor having provided thereon an adherent coating of an oxide of copper produced by oxidation of copper in an alkaline solution, said. oxide of copper having a thickness equiva lent to 0.3 to 2.0 milligrams of copper oxide per square centimeter of axial conductor surface, and a polyethylene composition filling the space between said conductors and being adherently united with said oxide-coated axial conductor.

References Cited in the file of this patent UNITED STATES PATENTS 2,379,318 Safford June 26, 1945 2,551,591 Foord May 8, 1951 2,593,922 Robinson et al. Apr. 22, 1952 OTHER REFERENCES Black Anodizing by McLean from Metal Finishing, June 1945, pages 247 and 248. (Copy inDivision 5 6 in 204-56.) 

1. AN ELECTRIC CABLE COMPRISING THE COMBINATION OF AN ELECTRIC CONDUCTOR AT LEAST A SURFACE LAYER OF WHICH CONSISTS OF COPPER, SAID COPPER LAYER HAVING PROVIDED THEREON AN ADHERENT COATING OF OXIDE OF COPPER PRODUCED BY THE ANODIC OXIDATION OF COPPER IN A ALKALINE SOLUTION, A SPACED ENCLOSING CONDUCTOR SURROUNDING SAID OXIDECOATED CONDUCTOR AND AN ELECTRIC INSULATOR COMPRISING POLYMERIZED ETHYLENE FILLING THE SPACE BETWEN SAID CONDUCTORS. 